Two-dimensional (2D) ultrasonic probes are necessary to support three-dimensional (3D) electronic, volume data acquisition for many clinical applications. State-of-the-art one-dimensional (1D and 1.5D) probes which electronically scan only in azimuth provide the 2D ultrasound images (azimuth and range) which are commonly used today. 2D probes scan electronically in elevation as well as azimuth, to provide a three dimensional data cube (azimuth, elevation and range) which can be processed using image processing software to produce a variety of image formats. These formats include conventional planar images, planar images at arbitrary scan planes, as well as representations such as surface rendering and orthographic presentations. Four-dimensional (4D) representations include 3D animations where the 3D rendering is updated in time.
Two-dimensional sensors are employed in other imaging modalities such as CT-scanners, and in other fields such as radar; and hence are well understood conceptually. Practical difficulties arise with the ultrasound modality due to the small, elemental feature size (fractions of a mm) and the large number of channels typically needed. These difficulties have stalled the introduction of fully electronic, 2D, ultrasonic probes.
Ultrasound systems today make use of a variety of 1D and 1.5D ultrasonic arrays. A 1D array has a fixed elevation aperture which is focussed at a fixed range, and is usually realized with a mechanical lens of sorts. A 1.5D array, on the other hand, has a variable elevation aperture, shading and focussing, but they are symmetric about the centerline of the array.
1D array transducers contain several tens or even hundreds of elements typically arranged linearly. The transducer elements 10, 12 may be arranged on a straight line (linear array) or a curved line (curved linear array or simply curved array) as shown in FIGS. 1A and 1B, respectively. The operation of a linear array or curved array are similar, the main difference being that the image expands with range (depth) for the curved array. A typical linear or curved array could have anywhere from 64 to 512 (or more) elements, depending on the cost and the application. The azimuthal spacing of elements is typically between half a wavelength and one wavelength. The elemental size in the elevation dimension is much larger, typically tens of wavelengths. The operating frequency is typically somewhere between 2 MHz to 20 MHz, depending on the clinical application.
Let's consider an example, where a 7.5 MHz curved array of the type shown in FIG. 1B has 256 transducer elements 12 in azimuth spaced by one wavelength; and the dimension of an element in elevation is, say, 40 wavelengths. At 7.5 MHz, the wavelength, λ, in tissue is about 0.2 mm. Therefore, the array spans about 51 mm in azimuth and 8 mm in elevation.
A narrow beam is created in the azimuth dimension by focussing the transmitted and receive energy along a particular beam or scan line 14, 16, as illustrated in FIG. 2A and FIG. 2B. Scanning is performed in azimuth (i.e. in a single elevation plane) using one of two schemes, sequential scanning or phased-array scanning. With sequential scanning, any given beam line is offset from all of the other beam lines in the azimuth direction. If the array is linear (rather than curved), the beam lines 16, 18 are parallel to one another (FIG. 3A). With reference to FIG. 2B, the central beam line 16 that is illustrated is shifted (offset) to the left or right with different offsets 20 to create a set of beam lines 22 that spans the region or volume to be imaged, as illustrated in FIG. 3A. Phased-array scanning, on the other hand, is achieved by rotating the central beam line 24 illustrated in FIG. 3B in azimuth, to the left and to the right, by a set of angular offsets 26. The beam lines 28 of the resulting set 30 of beam lines intersect at a common apex 32 (which may actually occur behind the array), and separate from each other as a function of range, as illustrated in FIG. 3B.
Premium probes generally employ wideband waveforms to achieve the fine resolutions needed in range. As a result, beamforming is done by adjusting time delays (in the narrowband waveform case, phases are adjusted rather than time delays) at each element used on transmit and receive. For a given pulse, a focal point is set along the range dimension. Appropriate time delays are used on the elements involved in transmission, so that their respective acoustic energy arrives at the specified focal range, along the specified beam line, at the same time. As a result, the waveform is said to be focussed at this point. On receive, time delays are dynamically applied to the elements involved in reception, to focus the received energy at each range.
Generally speaking, focussing is needed only in the near field of the array, where the ultrasound wave cannot be assumed to be planar, as it is in the far field. If one looks closely at the effect of this focussing operation in the azimuth and elevation spatial dimensions, one notices a difference. In azimuth, numerous transducer elements are available, each with a respective time delay to adjust dynamically with range on receive. The result is the azimuth resolution of the beam can be generally maintained uniformly with range as illustrated in FIG. 2A for a 1D linear array. There are no delays to adjust in elevation, however. As a result, a typical, fixed, lens-like beam pattern results in the elevation dimension, with the best elevation resolution occurring at the transmit focal point (in range), and with a degradation of the elevation resolution as one moves away from this focal point in range. This effect is also illustrated in FIG. 2A. The image plane thickness (i.e. in the elevation dimension) in effect varies with range for a 1D linear array.
The 1.5D array provides a solution to the image thickness problem, and therefore produces higher-quality, planar images than the 1D array (Wildes, D. G., et al., “Elevation Performance of 1.25D and 1.5D Transducer Arrays”, IEEE Transactions on Ultrasound, Ferroelectronics and Frequency Control, Vol. 44, No. 5, September 1997, pp.1027 to 1036). By using multiple rows of elements in the elevation dimension, as illustrated in FIG. 2B, multiple elevation lenses can be effected, each focussed at a different focal range. This is achieved by varying the time delays (through switching or otherwise) applied to the elevation elements while the acoustic signals are being received. In addition, a lens is typically used in the elevation dimension to help control the elevation focus. The net effect is that the elevation thickness (resolution) is maintained with range, thereby improving image quality. This is illustrated in FIG. 2B.
In a typical 1.5D array, each element might be λ×4λ (i.e. azimuth by elevation) in dimension. For an array with 128 elements per row and 8 rows of elements, the elevation dimension is 32λ or 6.4 mm and the azimuth dimension is 128λ or 25.6 mm at 7.5 MHz.
Consider the linear 1.5D array shown in FIG. 3A, containing 256 elements 34 in azimuth. Now 128 sequential beams 18 are typically used to form a rectangular, azimuthal image plane by scanning in azimuth as illustrated in the figure. Typical transducer dimensions for this state-of-the-art array are also indicated. (Note: only 16 columns of elements 34 are shown in FIG. 3A, for simplicity, where in fact, 256 elements are represented in the azimuth dimension).
A state-of-the-art array with λ/2 spacing in azimuth to support phased-array scanning is illustrated in FIG. 3B. This type of array produces pie-shaped images in contrast to the rectangular images produced using sequential arrays.
Unlike 1.5D arrays which are commonly found in premium ultrasound systems, 1.75D arrays are not yet in use in commercial systems (Puyun Guo, Shikui Yan and Quing Zhu, “Elevation Beamforming Performance of a 1.75D array”, IEEE 2001 Ultrasound, Ferroelectronics and Frequency Control Conference). 1.75D arrays are like 1.5D arrays, except there is no symmetry constraint. As a result, it is possible to provide a little bit of elevation steering. However, due to the large element size in elevation (several wavelengths), grating lobes become serious if the electronic scanning is significant (Puyun Guo, Shikui Yan and Quing Zhu, “Elevation Beamforming Performance of a 1.75D array”, IEEE 2001 Ultrasound, Ferroelectronics and Frequency Control Conference).
Interest in 3D Ultrasound is growing and all major ultrasound companies are paying attention. There are two ways that scanning is currently performed: sequential scanning and phased array scanning. It is common knowledge to those skilled in the art that if one conventionally-extends a 1D phased array (typically with λ/2 element spacing) to two dimensions (of equal size), or a 1D sequential array (typically with λ element spacing) to two dimensions, then data cubes could be acquired by 2D scanning, and the fine (e.g. an F number of 2, denoted herein as F/2) azimuth resolution currently available extends to elevation as well. Two fundamental difficulties, however, arise:                1. the cost is prohibitive;        2. the frame-time to acquire a 3D volume is far greater than the time it takes to acquire a 2D image.        
Consider extending a linear array with 256 elements (maximum of 128 used on receive) to two dimensions. The number of elements increases to 256×256=65,536. Transducer design/fabrication is very difficult, if not impossible, today. The number of receiver channels would also increase by a factor of 128 in order to provide the same resolution in both dimensions, all else being equal, while not increasing the number of shots (and hence acquisition time) needed per vector. Since system cost is proportional to the number of channels, the resulting cost is unaffordable.
Finally, it takes longer to acquire the data cube (as compared to the tens of milliseconds needed to acquire a conventional 2D image plane) since there are many more beams needed to interrogate the volume. At least 128×128=16,384 beams are needed, for each transmit focal range, with about 100 μs two-way time needed for each shot (this assumes a 10 kHz firing rate and a 7 cm depth needed). For two focal ranges, this implies an acquisition time of 3.2 seconds, assuming the number of channels available equals the number of elements used in the beamformer.
The aforementioned difficulties require practical trade-offs and novel solutions if 2D arrays supporting 3D electronic, volume data acquisition are to be an affordable reality.